The Life of a Fern.[1]
[1] For this Chapter I am indebted to my friend Mr. William Whitwell,
of Oxford.
One of the most marvellous of "the fairy tales of science" has now to
engage our attention for a time. The growth and fertilization of the
seeds--more properly called spores--of ferns, present phenomena of
remarkable singularity and interest. Growth is advisedly named first, as
in the present instance it really does occur before fertilization, which
is not the primary event in the life-history of a fern.
But a few words must be devoted to the preliminary question: What is a
fern?
The vegetable kingdom is divided into two great provinces, allotted
respectively to the flowering and the flowerless tribes. The flowering
plants have several distinct and visible organs for the formation and
fertilization of their seed, to each of which is assigned a special and
necessary office. In the flowerless section, on the contrary, there are
none of these visibly separate agencies in reproduction, and what are
usually termed the seeds do not show any parts representative of the
developed product. In the true seeds, which belong to flowering plants
alone, are contained the rudiments of a stem, leaves, and root, but in
the spores of the flowerless plants nothing of the kind is found. The
spores, again, are microscopic, while the smallest of true seeds can be
not only seen but easily picked up. You have, doubtless, met with the
peculiar fungus called a puff-ball, and amused yourselves by watching the
little clouds of impalpable dust which are shaken from it on the
slightest motion. Those fine clouds, not nearly so visible as a film of
candle smoke, are composed of innumerable spores, and such are the
representatives of seeds in every member of the great section of the
flowerless plants.
Now it is peculiar to ferns, that the cases in which these spores are
enclosed grow directly from the veins of what is usually called the
leaf, but is more correctly termed the frond, and always appear upon the
back or at the margin.
Ferns, then, are flowerless plants which bear their spores in cases
growing upon the back or margin of the leaves.
In order that the phenomena of growth and fertilization in ferns may be
clearly understood, it is necessary to refer to the process as taking
place in flowering plants. The tulip is most appropriate for an
illustration, inasmuch as its various parts will be recognised with
ease.
At the bottom of the blossom is a thick green oval body called the
ovary, which afterwards becomes the seed-vessel. At the top, this
narrows into a short column, surmounted by a three-cleft knob. Between
the ovary and the gorgeously painted flower-leaves are six curious
organs, termed stamens, consisting each of a long and rather slender
stalk, and a head formed somewhat like a hammer.
If the green oval ovary in the centre is cut in two, it will be found
divided into three chambers, in one or another of which, not usually in
all, will be seen a row of little knobs or buttons attached to the
partition in the middle. These little buttons are ovules, or seed-germs,
and the special office of the ovary is to produce these germs, and to
contain them until their full development and complete ripening into
seeds. But if the knobs are left just as they are, unfertilized, they
can never become seeds, and the plant will fail to reproduce its kind.
Turn we now to the stamens. Each of their hammer-like heads has two
chambers, full of beautiful little grains which are called the pollen.
Each grain is tastefully and delicately marked, and holds a transparent
watery fluid, in which a number of extremely small solid particles are
floating. What is required for the fertilization of the seed-germs
is--that this fluid should be conveyed to and taken up by them. But they
are in the centre of the thick green ovary--this in the chambers of the
stamens!
A simple arrangement brings all about. At a certain time we may see the
black heads of the stamens covered with a fine flour, which adheres to
whatever touches them. This flour is made up solely of pollen-grains,
escaping in unimaginable numbers from the chambers where they are
produced. At the same time the knob which crowns the seed-vessel puts
forth a thick and gummy ooze. The stamens are just long enough for their
heads to rise a little above this knob, upon which the pollen, when
escaping as I have stated, falls in great quantity, and is there held
fast.
Each grain then begins to swell, and to sprout (as the Rev. J. G. Wood
has it) something like potatoes in a cellar. All the sprouts, however,
pierce the knob, and push downwards until they reach the seed-germs
underneath. Each sprout is a tube of extreme minuteness, and when it
reaches a germ, attaches itself thereto, and, through the channel so
formed, the fluid is drawn out of the pollen-grain and absorbed by the
embryo seed. Fertilization is thus effected, and the growth and
development of the germ proceeds until it becomes a seed fully able,
when planted, to reproduce a tulip.
[Illustration: FERN SPORES.]
In ferns, the spores ripen and are ready for dispersion and partial
growth without any process of the kind. But, in truth, fertilization is
as necessary to the continuance of ferns as to the perpetuation of other
plants. The main difference lies in this: that the means of
fertilization, and the real germs of new plants, are produced from the
spores after they begin to grow.
When a spore falls upon a proper place for its development, a portion
of the outer membrane begins to swell, and a tongue-shaped projection is
formed, which becomes a sort of root. The one chamber of the spore
gradually subdivides, and becomes two, four, and so on, until for the
simple spore we have a tiny leaf-like expansion, now known as the
_prothallium_, or representative of a leaf.
Further than this the spore alone has no power to go, and the
prothallium is not truly the germ of the future plant. True germs,
needing fertilization, are produced upon it, and also the means whereby
they can be fertilized. These can be distinguished only by use of the
higher microscopic powers. If a portion of the prothallium is examined,
it will be found studded with little bladders, containing round
semi-transparent bodies of a greenish hue.
There may also be seen, though in fewer numbers, pellucid cells of an
entirely different character, consisting apparently only of a fine
membrane, forming an angular chamber, shaped in some instances like a
lantern of extreme delicacy and elegance. From the top of this chamber a
funnel-like shaft descends to a little germ which is situated at the
bottom. This germ is the real original of the future plant, and the
round bodies in their little cells, just before described, are the means
whereby it is to be fertilized and receive energy to develope into the
perfect fern.
But how can the needful contact between the germs and the fertilizing
bodies be brought about? Observation and experiment supply a strange
answer to this question.
The round bodies in the tiny bladders acquire a spiral or shell-like
form when they become mature. If a drop of water is then placed in
contact with the bladders, their contents will suddenly escape,
retaining for a moment the coiled appearance, but quickly lengthening
and partially unrolling.
By means of hairs with which they are furnished, and which at once
commence a ceaseless jerking motion, they forthwith launch out into the
water, and conduct themselves therein more like creatures endowed with
conscious life than mere organs of a settled and sedate member of the
vegetable kingdom.
These bodies, drawing near the germ-cells in the course of their travels
through the, to them, vast ocean of the water-drop, have been seen
arrested in their progress and passing down the funnel-shafts to the
germs below--so fulfilling the purpose for which they were designed and
their curious swimming powers were given.
The germs, so fertilized, become the underground stems of which I have
yet to speak, putting forth roots and producing the tender, rolled-up
buds which finally expand into the fronds whose grace and beauty we so
much admire.
These germs, appearing on the prothallium or leaf-like expansion of the
spore, are the true representatives of seeds, and the swimming bodies
correspond to the pollen or fertilizing dust of flowers.
Thus we see that germs and means of fertilization are produced in the
fern as truly as in higher plants, and that the simple agency whereby
the one may reach and exert the needful action upon the other, is the
_dew-drop_ resting on the prothallium from which they are developed.
Without the dew-drop or the rain-drop as a means of communication both
must perish with their mission unfulfilled. This is, perhaps, one of the
most singular instances ever to be found, of the mutual dependency of
created things, or, to give different expression to the same idea, of
the mode in which each link of the great network of existence is
connected with every other.
Returning to the fern, whose "strange eventful history" we have traced
so far,--the germ enlarges and becomes what is usually called the root,
but is really an underground stem. The true roots are the little
fibres--often black and wiry, looking more dead than alive--which
descend from this.
The stem may be of two kinds--long, thin, and creeping, as in the common
polypody, or short, stout, and upright, as in the common male fern.
At intervals along the creeping stem, or arranged more or less regularly
around the crown of the erect stem, little buds appear, which eventually
form the fronds which are the really conspicuous portion of the plant,
and whose aspect is familiar to us all. The buds present a character of
great interest and singularity. Instead of being simply folded together,
as leaves generally are,--in all but two of our British kinds the fronds
are rolled up after the fashion of a crosier or shepherd's crook. In
divided fronds, the sections are rolled up first, and singly, and then
the whole are rolled up again, as if forming but a single piece. The
aspect of some of these young fronds--in the common bracken, for
instance--with their many divisions all partially unrolled, is often
highly curious.
But in this I am proceeding too far. The first crop of fronds, even in
those kinds which when mature are most deeply cut, are usually very
simple in form--almost or wholly undivided.
This fact is often a source of great confusion to beginners. I well
remember two perplexities of the kind in which I was involved during the
earlier season of my attention to this subject.
Growing upon a rock by the roadside, I found a small fern, more
exquisitely beautiful than any I had seen before. I gathered and
preserved it, but for many months was wholly puzzled as to its nature.
Fancies arose that I was the happy discoverer of a new species,--and
what if Professor Lindley or Sir William Hooker were to name it after
me--Asplenium, or Polystichum, or something else, Meredithii? That would
be better than a peerage.
These were but fancies, and I was well pleased when further
experience--for books helped me not at all--showed that it was a young
plant of the common lady-fern. It was divided once only--into simple
leaflets--while the fully-developed frond of the matured plant is one of
the most highly subdivided our islands can produce.
When I began collecting ferns, I had not seen a specimen of the rare
holly-fern, and it was pardonable in me on finding some fronds which
evidently belonged to the shield fern genus, and were divided into spiny
leaflets only, to refer them to this species and tell a friend that I
had made a great discovery. But on going to the same plant a year later,
my mistake was made plain, as the new fronds were much more divided, and
showed the plant to be of the common kind, the prickly shield-fern.
On the rocky sides of little Welsh and Highland rivers, in glens where
the sunlight seldom enters, complete series of this fern in all its
stages--from the tiny simple leaf to the deeply-cut and boldly-outlined
frond of nearly three feet in length--may easily be obtained, and will
beautifully illustrate its varied and increasingly-divided forms.
Some fronds of course, as those of the graceful hart's-tongue, are
undivided even at maturity, except in occasional instances in which,
like creatures endowed with more sentient life, they become erratic,
and show a disposition to pass beyond the ordinary limitations. Curious
examples of tendency to a greater than even their proper large amount of
subdivision are occasionally shown in specimens of the lady-fern, which
become forked at the extremities not only of the fronds but of the
leaflets also.
The manner in which the fronds divide into lobes, segments, leaflets,
and so on, is of course largely dependent upon the character of the
veining, which differs widely from that of the flowering plants. In
these, the veins are either netted or parallel, but in ferns they are
forked, each branch again forking, and so on outward to the margin. This
is only partially true of the scale-fern, and not true at all of the
adder's-tongue; but it is the case with all other of our native kinds.
[Illustration: SCALY SPLEENWORT OR "RUSTY BACK."]
Passing now to the production of the spores, and so completing the cycle
of a fern's existence,--these appear in cases which spring in some
instances from leafless veins or central ribs, but mostly from the veins
as they usually occur, and at the back or, in the bristle-fern and
filmy-ferns, at the margin of the fronds. The cases grow in clusters
which are termed sori, each of which is generally protected by a
covering, though in the genus of the polypodies this is entirely absent,
the clusters being fully exposed to the diversities of wind and weather.
In the protected kinds, the cover assumes various forms. The filmy-ferns
have it as a tiny cup, enclosing the spore-cases. In the bladder-fern it
is like a fairy helmet. The shield-ferns, as their name implies, produce
it as a little shield, fastened by its centre. In the buckler-ferns it
is kidney-shaped, in the spleenworts long and narrow, and so on. Some
kinds can scarcely be credited with the formation of a real cover, but
their sori are protected by the turned-down margins of the fronds. In a
few sorts, separate fronds are provided for the production of the
spores, and these mostly differ in shape from the ordinary or barren
fronds.
The spore-cases are generally almost microscopic, flask-like in shape,
and encircled by an elastic ring of peculiar structure, which passes
either from top to bottom like a parallel of longitude, or round the
sides like the equator round the earth. The exact nature of this
band,--whether its elasticity be due to the mechanical arrangement of
its cells, which are narrower on the inner than on the outer side, and
apparently filled with solid matter, or to a quality of its
substance,--I am unable to determine.
[Illustration: WILSON'S FILMY-FERN.]
[Illustration: TUNBRIDGE FILMY-FERN.]
When the spores are fully ripe, and ready for dispersion, the band,
which has hitherto been bent around them, springs open with great
suddenness and force, tearing the enclosing membrane and casting them
forth upon the breeze, to undergo in their turn all the changes we have
traced, or, as must be the case with multitudes, such are the countless
numbers in which they are produced, to perish, humanly speaking, with
all the beautiful possibilities of their nature for ever lost.
The botanist is led away from care, not merely into holes and corners--
"Brimful dykes and marshes dank"--
but to glorious vales and to mountain tops, where fresh health-laden
breezes play around him, and he can delight in scenes of grandeur and
loveliness to a degree which only a true lover of nature knows.
A poet I have read gave sweet expression to thoughts and feelings which
I have often shared, when he wrote thus:--
"Oh! God be praised for a home
Begirt with beauty rare,
A perfect home, where gentle thoughts
Are trained 'mid scenes so fair;
"And where (God grant it so) the heart
That loves a beauteous view,
The while it grows in truth and taste
May grow in goodness too.
"For 'tis my creed that part to part
So clingeth in the soul,
That whatsoe'er doth better one,
That bettereth the whole.
"And whoso readeth nature's book,
Widespread throughout the earth,
Will something add unto his love
Of wisdom and of worth."
Happy are those who can find relief from the worry and turmoil of
business in the observation and study of the myriad forms of life which
flourish upon the earth, or whose record is laid up within its rocks.
But blessed is he who, from the contemplation of objects so varied,
wonderful, and beautiful, can with a full heart look upward to a God
reconciled in Christ, and in reverential and loving worship exclaim, "My
_Father_ made them all!"